The invention relates to new and useful improvements in radio frequency signal transmission, and more particularly to improvements is radio frequency diversity transmission, and more particularly to radio frequency signal transmission by any means or mode which symmetrically places a signal and its diversity copy about a common center frequency, and most particularly to such transmission using multiple tones.
Radio transmissions are subject to many vagaries, both natural and man-made, which can prevent completion of a communications circuit. One strategy used to overcome this problem is termed "diversity" communications. A common type of diversity communications is frequency diversity which involves transmitting the same information via multiple frequencies. In this way one of the frequencies may not be subject to atmospheric effects or intentional interference, and the communications circuit can be successfully completed.
One common form of diversity communications involves using the independent (ISB) mode and imposing the same information on both the upper and lower sidebands. Often the information to be transmitted is in digital form and is thus transmitted in the form of multiple discrete tones. Examples of such transmission techniques include Kineplex, frequency division multiplexed frequency-shift-keying (FSK) and phase-shift-keying (PSK). In these methods identical baseband audio is applied to both the upper sideband (USB) and lower sideband (LSB) channels of an HF transmitter to achieve diversity operation.
One of the unfortunate side effects of such ISB techniques is high peak RF voltage relative to the average power. Since HF power amplifiers are peak voltage limited, the transmitter gain must be reduced in order to prevent overdriving which causes distortion and interference.
It is therefore an object of the present invention to provide more reliable diversity transmission of information via radio signals.
It is a further object of the present invention to provide means to permit an increase in the average output power in a multitone diversity radio transmission system.
It is another object of the present invention to reduce the peak-to-average power ratio in a multitone diversity radio system.
Still other objects will become apparent in the following summary and description of a preferred embodiment of the invention.